The present invention relates to a method of automatic exposure control for an electronic still camera and an apparatus therefor.
Electronic still cameras have been provided that photograph an image and record the image in a still video floppy disc instead of on a silver salt photographic film. In such an electronic still camera, image signals are picked up though a solid state imaging device such as an interline transfer CCD (charge coupled device) image sensor, which will be hereinafter referred to as a CCD. In order to determine the exposure value, a photosensor such as a SPD (silicon photo diode) has generally been used, but it is also known, for example from Japanese Unexamined Patent Publication 63-169180, to calculate the exposure value based on charges output from a CCD.
However, the dynamic range of a CCD, that is, the brightness range detectable by a CCD is about 8 EV (exposure value) that is narrower than that of a SPD, e.g. about 21 EV, which supposes that the film speed is ISO 100. Therefore, in view of the fact that the subject brightness variation is generally about 5 EV but may sometimes exceed such an ordinal level, the narrower dynamic range of the CCD will sometimes cause an erroneous measurement.
So as to avoid the erroneous measurement, a conventional electronic still camera measures again the subject brightness after adjusting aperture size and shutter speed to an exposure value determined by a calculation based on a first brightness value that has been measured immediately after the shutter release operation, calculates a new exposure value based on the secondly measured brightness value and compares it with the first exposure value so as again to adjust aperture size and shutter speed to the new exposure value. This procedure is repeated until an optimum combination of an aperture size and a shutter speed is determined.
This method is disadvantageous in that, since it takes a relatively long amount of time especially to adjust the aperture size, the time lag from the shutter release operation to the practical start of exposure becomes larger. As a result, an miss an optimum shutter chance.